Air separation plants operate at high pressures and have potentially hazardous equipment that should be considered for enclosure by barriers. The primary purpose of a safety barrier is to prevent injury to personnel. A secondary purpose is to prevent or lessen damage to adjacent equipment. Of particular concern are barriers where oxygen services are involved. Oxygen flash fires and/or blowouts are much different in temperature, intensity and fuel from a 48 hour petroleum refinery or petrochemical plant fire. In refinery or petrochemical fires, the fuel is hydrocarbons exhausting through metal such as pipes and tanks, and temperatures can reach as high as 1800° F. In an oxygen fire, the fuel is metal itself. These fires burn at extremely high temperatures of 5500° F. and higher.
The Industrial Gas Council (“IGC”) of the European Industrial Gas Association has published document 1142-6829, 27-01-E, “Centrifugal compressors for oxygen service: code of practice.” According to the IGC, an oxygen fire most often starts in areas of high component or gas velocity. The most likely sites are around the impeller or recycle valve. Bum through is most likely to occur at places close to the seat of the fire where gas pressure is high and the thermal mass is small, such as the compressor casing, the compressor shaft seals, expansion bellows adjacent the casing/volute, first and second bends in the process pipework upstream and downstream from the compressor flanges, and the recycle valve and its associated outlet pipe and first downstream bend. A jet of flame is accompanied by a widening spray of molten metal which splatters over a wide area. A blast can collapse a barrier if the barrier is not engineered to withstand the blast in the context of its surroundings. The release of pressure and the rotational energy of a centrifugal rotor accelerates projectiles which either pass through holes burned in the compressor casing or rip holes in the casing and go on to hit the safety barrier.
Thus, in general, there are three types of mechanical forces to be considered in designing a barrier: (i) force of a jet resulting from release of compressible fluid as from a hole in a compressor casing or pipework accompanied by molten metal issuing from a hole, (ii) an overpressure or blast force from massive release of stored energy, such as stored inventory of oxygen or hot combustion gases, and (iii) force resulting from the impact of steel projectile blast fragments traveling a high velocity as a result of a blast. Further, design of a safety barrier to withstand an oxygen fire must enable it to standup to thermal loads of 5500° F.-plus temperatures at the location of metallic combustion and molten metal splatter.
In addition to IGC document 27/01/E, another relevant reference is Oxygen Compressor Installation and Operation Guide, a standard for current industrial practices for oxygen compressor installations, operations, maintenance, and safety, publication number G-4.6 (1992), issued by the Compressed Gas Association (“CGA”), 4221 Walney Road, 5th Floor, Chantilly, Va. 20151-2923. Both these guides allude to fragmentation safety barriers around centrifugal compressors in oxygen service. The CGA document G4-4.6 at art. 4.2.2.1(c) makes reference to “the impact of system components that may be ejected” but does not specify a load parameter. The IGC document 27/01/E, re-issued in December of 2000, mentions a use of 30 kilograms (kg) at 50 meters per second (m/sec) irrespective of compressor duty. A mechanical load of 30 Kg at 50 m/sec is the equivalent of a 66 pound projectile moving at 164 ft/sec (112 miles per hour).
Air separation plants built and upgraded over the last 50 years are typically a composite of evolving engineering practices. Barriers and protective enclosures of the most recently engineered oxygen compressor installations are commonly erected using steel reinforced concrete. However, there are many existing indoor centrifugal oxygen compressors installed with barrier enclosures constructed of structural steel and undefined or untested insulation. These are of questionable strength and may not be in compliance with CGA G4-4.6; they have a questionable capacity to impede imparted fragments from a compressor failure.
Retrofit or upgrading of existing centrifugal compressor installations for barriers that would be in compliance with CGA G4-4.6 and Industrial Gas Council document 27-01-E presents numerous problems. Engineering solutions for an existing plant may be limited by the arrangement of the installed compressor and piping systems for which an upgraded barrier is to be erected. For example, in the case of centrifugal compressors, compressor rotating components typically are located on intermediate or mezzanine stories of buildings above heat exchangers and other pipework. The support columns for these upper levels may not have been placed—and the columns or the floor on which the rotating components are mounted may not have been engineered—to take the additional load of reinforced concrete walls added as a barrier upgrade to the mezzanine floor. Further, any barrier membrane capable of impeding a 66 pound projectile moving at 164 ft/sec would transfer the membrane load to the barrier structural steel, and the barrier structural steel would transfer that load to the existing structural steel of the building. If the barrier structural steel suffered a direct hit from the fragment, maximum loads would be transferred to the building structural steel. While fragment barriers intended for new construction can have building steel and concrete engineered to take any transferred blast fragment load, the same is not true of existing buildings; in them the building steel and concrete may not have the capacity to take the high transferred loads. Thus it is necessary to understand the maximum loading that structural steel barriers will receive and transfer.
In addition to constraints from existing building structure, use of reinforced concrete as an upgrade barrier material is often not feasible because of density and congestion of oxygen system installations, including compressors, compressor gas coolers and interstage piping, piping to and from a compressors including recycle piping, oil piping and other lubricant lines or service lines for rotating equipment, etc. The compressor and pipework may require location of protective barriers in locations where a reinforced concrete wall cannot be fitted.
Apart from ability to protect against blast fragments, a fire barrier for an oxygen system must be able to stand up to the terrific heat of an oxygen fire. While concrete can do this, to a point, the oxygen handling industry has had trouble in certifying other materials as able to withstand the heat of an oxygen fire. Many industry standards exist, but none represent the conditions of an oxygen flash fire where metal is a fuel and metal splatter burns through materials. The capacity of materials alternative to concrete to impede oxygen metal fires is largely unknown.
Wholly apart from serving as a fire barrier, a basic need exists for safety barriers and a method for designing them for air separation and other high pressure gas processing plants to be able to withstand a projectile impact of substantial size and velocity produced by sudden release of pressure, whether or not the pressurized gas is combustible, where use of reinforced concrete walls is not feasible. This includes the need to understand all possible dynamic loads from a blast fragment, and in respect to loads on a steel membrane, to understand energy absorbed by a deformed membrane, loads transferred from the barrier membrane to barrier structural steel, and loads transferred from the barrier structural steel to the building structural steel or concrete, since existing building structural steel might not accept the transfer of loads received by a structural steel barrier. Where the pressurized gas is combustible, such as oxygen, there is the further need that a safety barrier that is not steel reinforced concrete not only will stand up to a blast fragment of considerable mass and speed but also will stand up to extremely high temperatures without ignition or burn through. Advantageously, such safety barrier structures would be one suitable for use both in new construction and for retrofit and upgrading of existing plants. In the latter respect, a desirable fragment barrier system would not have undue weight and/or bulk and would not function so as to exceed safety margins of structural steel already in place in an existing building.